The companies named in the suit were No. 1 U.S. power producer American Electric Power Co. Inc., Southeast utility Southern Co., the Tennessee Valley Authority public power system and Midwestern power companies Xcel Energy Inc. and Cinergy Corp.

The lawsuit accuses the five companies' power plants of being responsible for almost a quarter of the U.S. utility industry's annual carbon-dioxide emissions and about 10 percent of the country's total.

It seeks, at minimum, a 3 percent reduction in carbon dioxide emissions each year for 10 years.

The power companies being sued said they are striving to cut emissions and had programs in place to cut even more.

The group of attorneys general filing the suit include New York Attorney General Eliot Spitzer, who in the past has sued utilities running coal-fired plants and accused President Bush's administration of not enforcing clean air regulations.

"The federal government has abdicated responsibility on this matter and that is why we are filling the breach," Connecticut Attorney General Richard Blumenthal said at the news conference. He drew a parallel to the states' suit against big tobacco companies.

"For those who say we may fail, I say think tobacco. As in tobacco, we have a uniquely dangerous public health threat on our hands."

The other states involved in the suit are California, Connecticut, Iowa, New Jersey, Rhode Island, Vermont and Wisconsin.

Two land conservation groups, New Hampshire Audubon and Open Space Institute, filed a separate, similar public nuisance case against the same five companies yesterday, said the Natural Resource Defense Council, an environmental group representing both organizations.

Both suits were filed in federal district court in Manhattan.

"Much has been done already in the United States and by our industry on carbon and more is expected to be done," a Southern Co. spokeswoman said. "There are no simple answers to this issue. It requires long-term planning."

An Xcel spokesman said the company is in the midst of some of the largest voluntary emissions reductions programs including a $1 billion program to cut emissions from the twin cities area. It also plans to triple its portfolio of wind generation in the next eight years.

An AEP spokeswoman said filing lawsuits is not a constructive way to address the emissions issue and that the company has committed to cap and reduce its carbon dioxide emissions by a cumulative 10 percent by 2006.

But the states are not impressed.

"We are aware of what the companies have been doing and obviously voluntary actions so far have been inadequate," Spitzer said at the news conference. (Additional reporting by Carolyn Koo, Richard Valdmanis and Timothy Gardner)